It is known to use multi-cylinder air compressors on freight and passenger locomotives to supply compressed air to various locomotive systems, such as the operating and control equipment of a railway air brake system. Prior art techniques for servicing the air compressor system have essentially required uninstalling and shipping major components of the air compressor system, such as the entire compressor, to a specialized compressor servicing site. This approach may lead to unnecessary costs and delays, if the type of component causing the malfunction was one that could be replaced in-situ at the locomotive (i.e., as installed onboard the locomotive) without having to incur the delays and expenses associated with shipping the entire compressor to the specialized servicing site. However, heretofore there was no effective procedure or test apparatus to diagnose locomotive air compressors in-situ to determine if the malfunction was due to an in-situ serviceable component or to a cause that required removal of the air compressor system and servicing off-board of the locomotive.